Field of the Invention
The present invention relates to an image pickup optical system, which is suited to be used for an image pickup apparatus such as a silver-halide film camera, a digital still camera, a digital video camera, a monitoring camera, and a TV camera.
Description of the Related Art
In recent years, an image pickup optical system used for an image pickup apparatus is required to have high optical performance, a wide field angle, a large aperture ratio, and the like.
In each of Japanese Patent Application Laid-Open No. 2013-29658 and Japanese Patent Application Laid-Open No. 2013-7853, there is disclosed an image pickup optical system in which, in order from an object side to an image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and including an aperture stop, and a third lens unit having a negative refractive power are arranged.
The image pickup optical system has a refractive power arrangement that is symmetrical with respect to the aperture stop so that off-axis aberrations such as a lateral chromatic aberration, a field curvature, and a distortion, which are caused by the first lens unit, are corrected by the third lens unit. In this manner, good optical performance is obtained. Moreover, in the image pickup optical system, the second lens unit is configured to move to perform focusing.
With the image pickup optical system having the symmetrical refractive power arrangement, it is easy to correct the off-axis aberrations. On the other hand, when an incident angle of a light flux that has passed through the image pickup optical system with respect to an image pickup surface becomes larger, a resolving power is reduced. In particular, when the wide field angle of the image pickup optical system is to be realized, this tendency becomes stronger. In order to reduce an incident angle of an off-axis light flux with respect to the image pickup surface, the third lens unit may include a negative lens and a positive lens, which are arranged in the stated order from the object side, for example.
With this configuration, the incident angle of the off-axis light flux with respect to the image pickup surface may be reduced. Moreover, in order to obtain the high optical performance over an entire image plane, a cancelling action between the aberrations caused by the first lens unit and aberrations caused by the third lens unit needs to be performed satisfactorily. To this end, it is important to appropriately set the negative refractive powers of the first lens unit and the third lens unit, for example.
In the image pickup optical system disclosed in Japanese Patent Application Laid-Open No. 2013-29658, the positive lens included in the third lens unit has a strong positive refractive power so that the incident angle of the off-axis light flux with respect to the image pickup surface may be reduced, but it is difficult to perform the cancellation of the aberrations with the first lens unit in a balanced manner. In addition, a configuration in which an on-axis light flux diverged by the first lens unit enters the second lens unit including the aperture stop is adopted, and hence when an aperture of the image pickup optical system is increased, various aberrations such as a spherical aberration and a coma tend to increase. In the image pickup optical system disclosed in Japanese Patent Application Laid-Open No. 2013-7853, an entire lens length is short, but the refractive powers of the lens units are strong, resulting in increased generation of various aberrations, and in difficulty to obtain a high resolving power over the entire image plane.